/Engine/Source/Runtime/Core/Public/Containers/SortedMap.h
Move all items from another map into our map (if any keys are in both, the value from the other map wins) and empty the other map.
template<typename OtherArrayAllocator, typename OtherSortPredicate> void Append ( [TSortedMap](API\Runtime\Core\Containers\TSortedMap)< [KeyType](API\Runtime\Core\Containers\TSortedMap\KeyType), [ValueType](API\Runtime\Core\Containers\TSortedMap\ValueType), OtherArrayAllocator, OtherSortPredicate > && OtherMap )